Decorative and wear resistant coatings comprising Ti/Al and N are useful in a variety of applications. For example, such coatings may be used to coat stainless steel panels that are utilized to cover walls, pillars, ceilings, floors, doors, handrails, screens, and other parts of buildings to impart beauty and durability thereto. The coatings may be used to coat markers, pens, and the like, and for watch housing and wristlet coatings, as well as for other cosmetic jewelry applications.
Cathodic sputter processes are used to deposit such coatings by reason of the high production rates and precise coating controls resulting from these methods. Cathodic sputtering techniques are well known and need not be repeated here in detail. Suffice it to say that in accord with such processes, gas ions are caused to bombard a target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials traverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
Conventional methods of sputter depositing Ti/Al/N coatings employ a Ti/Al target prepared by either powder metallurgical or casting techniques. The Ti/Al/N coating is deposited by reactive sputtering of the Ti/Al target in a N.sub.2 atmosphere.
Prior art powder metallurgical methods include: (1) mixing the desired stoichiometric ratio of Ti and Al elemental powders, followed by cold isostatic pressing (CIP) of the blended powders to full density; (2) mixing the desired stoichiometric ratio of Ti and Al, prereacting the elemental Ti and Al powders to form intermetallic compounds (TiAl, TiAl.sub.3, etc.) followed by hot isostatic pressing (HIP) of the pre-reacted powders to full density and; (3) mixing the desired stoichiometric ratio of Ti and Al elemental powders, followed by simultaneous densification and reaction of the Ti and Al to form the intermetallic by HIPing of the blended and non-reacted elemental powders (reactive HIP).
In addition to the conventional casting and powder metallurgy techniques, U.S. Pat. No. 4,997,538 (Luthier et al) teaches production of a Ti, Al, O, and N target by one of three methods. In the first approach, TiO.sub.2 and AlN powders are sintered in a molar ratio of 1.5:2.0. A second alternative is to sinter TiN and Al.sub.2 O.sub.3 powder in a molar ratio of 1.5:1.0. Lastly, another alternative method provides for mixing of TiO.sub.2 and AlN powder in a molar ratio of 1.5:2.0 followed by cold pressing. No mention of achieved or optimal density is made in the patent and the method of mixing TiO.sub.2 and AlN powder, followed by cold pressing, does not yield a solid target.
The targets produced by Luthier are used in a cathodic arc sputtering system to deposit a black-colored decorative type coating on a substrate in an argon atmosphere. This disclosure is the only one known to the applicants in which a Ti/Al/N coating can purportedly be sputtered without the use of reactive N.sub.2 gas.
Another patent which may be of interest is U.S. Pat. No. 4,962,612 (Kuwano) in which Al/Ti alloy targets are prepared by a hot pressing method. The targets so produced are used in a sputter ion plating process in the presence of a mixed Ar/N atmosphere and optional C.sub.n H.sub.m gas component. The atmosphere is maintained under reduced pressure of 10.sup.-2 Torr. to 10.sup.-4 Torr.
The above noted target production methods are not devoid of problems. For example, in certain powder metallurgical methods calling for CIPing of blended elemental powders, when targets produced thereby are subjected to high operating powers, the target temperature can increase to a point where the target constituents undergo an exothermic reaction. This reaction can result in a number of undesirable effects including target swelling, spalling and melting. Consequently, damage to the sputtering apparatus itself may occur with attendant poor coating quality. Accordingly, such deleterious effects preclude the use of high sputtering power and its concomitant high coating deposition rates.
Similarly, targets produced by HIPing of prereacted powders or by reactive HIPing are intermetallic. That is, they consist of true TiAl, Ti.sub.3 Al or TiAl.sub.3 compounds. These intermetallics are brittle in nature and are difficult to machine into the desired shape. Moreover, the targets are subject to cracking problems.
On the other hand, targets produced by conventional casting methods exhibit compositional heterogeneity, segregational effects, grain size control difficulties and brittleness.
Accordingly, it is an object of the invention to provide a method for producing a versatile Ti/Al sputter target, which upon sputtering, can provide decorative and wear resistant coatings. It is especially desirable to provide a method for producing a target that can provide for variable target composition and grain size. The provision of a target which can be operated over a wide range of sputtering powers and temperatures while minimizing cracking and spalling problems attendant upon use of many of the prior art targets is also desirable.